permissive

Giving permission, or predisposed to give it; lenient.

"Permissive" working allows more than one train to be in a block section at one time but trains must be run at low speed in order to stop on sight behind the train in front. Such working is often authorised to allow freight trains to "bunch" together to await a path through a bottleneck instead of being strung out over several block sections, as would be necessary if absolute working were in force.

Open to the public by permission of the landowner.

That allows the replication of viruses.

A grammatical form indicating that an action is permitted by the speaker.